Holik H. (ed.) Handbook of Paper and Board

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lined with reinforcing sheets of paper. It is used in building construction for walls

and ceilings.

Gypsum can be sourced from either natural deposits or as a byproduct, for

example in the production of fertilizer or citric acid. Another gypsum byproduct

originates from the desulfurization of flue gases emitted by power plants operating

on the basis of fossil energy. Although available in huge amounts, so far all at-

tempts to make this byproduct technically and economically useable as paper filler,

have been unsuccessful. Gypsum may exist as different chemical structures

(CaSO

O dihydrate, CaSO

1/2H

O hemihydrate, CaSO

anhydrite) and in

general exhibits a high solubility, more or less independent of the pH environment

[11]. Figure 2.22 compares the solubility of gypsum and calcium carbonate versus

the pH.

2.2.1.5 Specialty Filler Pigments

2.2.1.5.1 Calcined Clay

This is a specialty filler, applied to the sheet mainly in order to increase light

scattering/opacity and to reduce the ink print-through potential. Typical areas for

the application of calcined clay are lower basis weight uncoated wood-containing

papers (i.e. telephone directories, low basis weight newsprint and SC paper etc.).

In the calcination process of kaolin, the water of hydroxylation (14%) is first

driven off at temperatures of 500–700 °C. On continued heating up to 900–1000 °C

the particles begin to fuse together into secondary particle aggregates, accompa-

nied by an increase in brightness. Further agglomeration results in tertiary parti-

cles. The final result is a large number of kaolin-air interfaces and relatively high

internal pore volume leading to increased light scattering and opacifying proper-

ties.

Fig. 2.22 Comparison of the pH related solubility of gypsum

and calcium carbonate (source: OMYA).

2 Raw Materials for Paper and Board Manufacture

2.2.1.5.2 Titanium Dioxide

This is a specialty and particularly effective filler. It is applied in specialty papers,

often at low basis weights, with particularly high demand in dry and wet opacity

and sheet brightness. TiO

can be found in thin print paper, i. e. bible paper, high

opaque grades, label paper and decoration paper.

Titanium is the fourth most abundant chemical element found on earth. In

nature, titanium occurs only in the form of oxides or mixed oxides with other

elements. Mineable deposits are generally of volcanic origin. The titanium dioxide

pigment industry uses between 90 and 95% of the global titania ore extracted.

Titanium dioxide is a synthetic material, commercially produced by the sulfate

or the chloride process. It exists in three modifications with different crystal lattice

structures and therefore different physical properties. The modifications are rutile,

anatase and brookite. Only rutile (hexagonal close packing of the oxygen atoms)

and anatase (cubic close packing of the oxygen atoms), which differ from each

other in some of their physical properties, are of technical importance. The typical

refractive index for rutile is 2.75 and 2.55 for anatase (cf. CaCO

1,58). In compar-

ison to other pigments TiO

exhibits also an extraordinarily high density

(3.9 g cm

–3

anatase, 4.2 g cm

–3

rutile). Rutile gives higher brightness (but less blue

whiteness) and opacity, while anatase supports optical brightening agents better as

it absorbs less ultraviolet light. Compared with the rutile form, anatase has a lower

Moh’s hardness of 5.5 to 6.0 instead of 6.5 to 7 [12].

Even at only low addition levels, TiO

has a remarkable effect on paper opacity

due to its unmatched high refractive index and high light scattering potential.

Because of its high refractive index, TiO

is the only pigment, which also provides

high opacity when the paper is wet (label paper). The extra high refractive index is

also important for providing opacity in the manufacturing of decoration paper. In

the production process of this specialty paper, resin penetrates completely

throughout the sheet structure, thereby filling up the voids and diminishing pore

structure volume with resulting reduced light scattering. Because of the extreme

fineness (average particle size 0.2–0.25 mm) and the complex chemistry in applica-

tion (for example high amounts of cationic wet strength agent etc.) TiO

filler

pigments can be difficult to retain in the paper web.

Due to its high cost, TiO

is often used in combination with extenders. These

extenders are other less costly minerals (for example Al(OH)

, synthetic silicate

and silica, barium sulfate), which are capable of replacing TiO

to a limited extent

while producing similar results. The effect of providing wet opacity to the sheet by

TiO

, however, cannot be reached by using extenders alone. No extender matches

the extra high refractive index of TiO

. The closest to TiO

in terms of refractive

index is zinc sulfide with a value of 2.37.

2.2.1.5.3 Amorphous Silicates and Silica

These are also specialty pigments, mostly used in lower basis weight newsprint

and directory paper. These pigments are added in small percentages (sometimes

applied in combination with regular paper fillers), to enhance paper brightness

and opacity as well as absorption properties, e.g. reduction of print-through.

2.2 Non-fiber Raw Material 53

Amorphous silicates are produced by destabilization of soluble silicates to yield

amorphous discrete particles in varying degrees of aggregation. The high specific

surface area in combination with an organophilic surface is responsible for the

excellent ink adsorption characteristics. Amorphous silicate also has the potential

to increase somewhat the bulk of the paper sheet.

In a recent development, Mg-Al-silicate was coprecipitated with a rhombohe-

dral-shaped precipitated calcium carbonate as core, as one single product. This

development has already found its way into commercial application. One target is

to reduce the ink print-through in lower basis weight newsprint or directory paper

at lower cost than regular Mg-Al-silicates [13].

2.2.1.5.4 Aluminum Trihydrate (Hydrated Alumina)

This represents a specialty filler which contributes to paper brightness, ink re-

ceptivity and acts as a flame retardant. The effect on flame retardancy can be

explained by the 35% of bound water based on material weight. The bound water

is released at temperatures above 150 °C. The raw material source for the produc-

tion of aluminum trihydrate (ATH) is bauxite. Bauxite is a blend of Al

, Fe

SiO

, H

O, TiO

and other minerals. To produce ATH it is necessary to stabilize

the alumina content and to separate out the other minerals. This is done by the so-

called Bayer process. After final filtration, the clear liquor of sodium aluminate is

seeded with specially prepared fine crystals of ATH. This seeding causes the so-

dium aluminate to decompose to ATH or Al(OH)

which forms a precipitate.

Some bauxite ore deposits exhibit a rather high purity and brightness. Such

material can be converted directly into filler grades. The physical difference be-

tween a fine precipitated ATH and a fine ground ATH is small.

2.2.1.5.5 Other Fillers

It must be mentioned, that there are several other industrial minerals being used

directly or indirectly as paper fillers. These materials include barytes, barium sul-

fate, calcium sulfite, zinc oxide, zinc sulfide, diatomaceous earth, mica, bentonite

and pyropholite. Synthetic organic fillers are, for example, based on polystyrene or

urea-formaldehyde. Local availability, cost and the need for obtaining certain spe-

cial properties are the deciding factors in the selection of these materials. However,

the total volume is quite small in comparison to the major fillers and specialty

pigments, as described above.

2.2.1.6 Outlook

There is a continuous demand for technical and economic development in the

various paper grades. In this context, the quality and choice of filler is also often

reconsidered. Brightness, opacity and surface properties like smoothness, uni-

formity, surface strength and printability will continue to be of great importance.

To address these demands, further specific pigment developments and the com-

2 Raw Materials for Paper and Board Manufacture54

bined application of different filler grades can be expected. Costructuring different

pigments and coprecipitation could represent suitable tools to reach some of the

functional targets. Investigations on how to further increase the filler loading in

various paper grades are constantly ongoing. Maintaining stiffness properties at

increased loading and cost effectiveness are some of the challenges. At the already

very high filler loading levels as practiced in Europe, each% point of further in-

crease represents a particularly significant success. Filling through the surface of

the sheet could be an additional tool for completely new developments. Positioning

the fillers at a specific location within the z-direction of the sheet (multilayering) is

in an early stage of investigation. Replacing more expensive specialty pigment

applications by regular fillers is always of interest. Calcium carbonate based fillers

will penetrate the paper market even further, as for example currently observed in

the area of higher brightness SC paper.

Much emphasis is given today on seeking the potential of nanotechnology in the

area of papermaking. It remains to be seen if nano per se will provide innovative

products, but there are materials available now, primarily in structured forms of

PCC or surface modified GCC that provide nanoscale features on the surface of

otherwise microparticles. Properties so far indicate high absorption potential, and

their use in digital inkjet printing and as specialty fillers are being evaluated in-

tensively [14, 15].

2.2.2

Coating Pigments

Werner Kogler

†

, Thoralf Gliese, and Werner J. Auhorn

2.2.2.1 General Overview

Coating improves the surface quality of paper and board resulting in higher bright-

ness, smoothness and gloss as well as better opacity and generally significantly

improved printability. Pigments are the main coating components for improving

the surface properties of coated paper and board grades. Usually they account for

80–95% of the total dry coating weight or, as a volume fraction, 70–80 % of the

solid material of the coating. This is based on a pigment density of about

2.5gcm

–3

and a density of the rest of the solid material in the coating of about

1.0gcm

–3

. In 2004 a total of approximately 18 V 10

tons of coating pigments were

used worldwide in the paper and board industry, which means nearly 2/3 of the

total usage of fillers and coating pigments. Kaolin (Clay) has been the biggest

coating pigment in the past, but its share dropped to 42% in 2004 while natural

ground calcium carbonate (GCC) took over the No. 1 position with 51% share.

Recently precipitated calcium carbonate (PCC) with a present share of 3% switched

from the status as a special coating. Pigments also largely determine the cost of the

coating. A sizeable number of white pigments are available for surface finishing in

the paper and board industry. A large majority of these are of natural origin, being

physically and chemically homogenous minerals formed by inorganic processes.

Organic products like plastic pigments may also be used in coating colors in spe-

cial cases. A more detailed description of the pigments used as fillers, their origin,

2.2 Non-fiber Raw Material 55

preparation and properties is given in Section 2.2.1 and a more detailed descrip-

tion of those used as coating is given in Chapter 7.

The pigments can be classified as:

• Main pigments:

– ground calcium carbonates (GCC)

– kaolin clays

– precipitated calcium carbonates (PCC) (recently switching from a special pig-

ment to a main pigment)

• Special pigments:

– talc

– gypsum (calcium sulfate)

– plastic pigments (e.g. polystyrene)

– satin white (calcium sulfoaluminate)

– barium sulfate

• Additional pigments:

– calcined clays (kaolins)

– titanium dioxides

– alumina trihydrates

Central properties of pigments are particle size and size distribution, particle

shape and shape distribution, refractive index, light scattering and light absorption

and density. The properties of coating color and the final coating can be influenced

and explained to a great degree by these factors. The following simplifies how

different quality parameters of coated paper can be improved by changing specific

pigment properties:

• better gloss by increase in platiness and/or decrease in particle size

• higher opacity by increase in refractive index and/or decrease in particle size

• higher brightness by decrease in light absorption

• higher porosity and ink absorption by decrease in packing (less compact pig-

ment layer), e.g. through mixing of different particle shapes

Fig. 2.23 Pigment to a main pigment.

2 Raw Materials for Paper and Board Manufacture

• higher bulk and better coverage by decrease in packing (decrease in density)

through mixing of different particle shapes.

Table 2.7 gives an overview and comparison of the most important physical and

optical properties of coating pigments. Besides the main pigments for paper coat-

ing, only the most important special and additional coating pigments are described

in more detail in this section (for others see Section 2.2.1).

2.2.2.2 Main Coating Pigments [16–19]

2.2.2.2.1 Kaolin Clays

Kaolin clays in paper coating have probably been used since the second half of the

19th century when paper coating was developed. Since that time they played a

dominant worldwide role as a coating pigment almost until today. Kaolin is one of

the most widely occurring minerals. Kaolinite, the principal constituent of kaolin

is a layered aluminosilicate having the chemical formula Al

(OH)

. Kaolin is

Table 2.7 Physical and optical properties of coating pigments

(main source: [16]).

Pigment Chemical

composition

Most particles

(mm)

Particle shape Density

(kg dm

–3

)

Refractive

index

ISO-

Brightness

Kaolin clay Al

·2SiO

·2H

O 0.3–5 Hexagonal

platy

2.58 1.56 80–90

GCC

CaCO

, MgCO

(2–3%)

0.7–2 Cubic, pris-

matic, platy

2.7 1.56–1.65 87–97

PCC

CaCO

0.1–1.0 Variable,

usually rodlike

2.7 1.59 96–99

Talcum MgO · 4SiO

·H

O 0.3–5 Platy 2.7 1.57 85–90

Gypsum CaSO

·2H

O 0.2–2 Roundish 2.3 1.52 92–94

Plastic pigment

– solid

– hollow

Polystrene most

common

0.1–0.5

0.4–1.0

Spherical

1.05

0.6–0.9

1.59

93–94

Calcined kaolin Al

·2SiO

0.7 (median) Aggregated

plates

2.69 1.56 93

Titanium dioxide

– anatase

– rutile

TiO

0.2–0.5

Rodlike

Roundish

3.9

4.2

2.55

2.70

98–99

97–98

ATH

Al(OH)

0.2–2 Platy 2.42 1.57 98–100

a Ground calcium carbonate (calcite), b Precipitated calcium carbonate,

c Alumina trihydrate.

2.2 Non-fiber Raw Material 57

an intrinsically valuable coating pigment because of its platy particle shape, good

color (white or near white), and the relative ease with which it can be processed to

a fine particle size.

2.2.2.2.2 Ground Calcium Carbonate (GCC)

GCC as a coating pigment was successfully introduced in 1973. It marked an

epochal step towards lower production costs and improved paper and print quality

by the successful development of “high solid” coating in the 1980s. Herewith GCC

allows reduced binder requirements and drying energy savings. The development

of finer-grade carbonates with particle sizes down to 99% < 2 mm and special

particle size distribution curves covers a broad spectrum of whiteness, paper and

print gloss, as well as opacity. High solids coating colors offer excellent runnability

even at the highest coating speeds of > 1800 m min

–1

. This also explains the over

proportional growth of ground calcium carbonate as a coating pigment during the

last three decades, whereas kaolin is stagnating and talc is showing constant

growth, especially for gravure coating colors.

2.2.2.2.3 Precipitated Calcium Carbonate (PCC)

PCC allows the production of coating pigments with specific morphologies by

controlled synthesis. Particle size, particle size distribution, and particle shape can

be controlled. The surface properties of the calcium carbonate particles can also be

changed if needed. Commercial PCC pigments typically have CaCO

contents

higher than 97%. The remainder is MgCO

(magnesium carbonate) and other

residues. Industrially, it has been found practical to produce PCC near the paper

mill by using what are called satellite plants. This saves inbound transportation

costs because the necessary CO

gas (40% of the final product weight) can be

taken from a local source and because calcium oxide can be delivered to the PCC

satellite plant in a dry form. Outbound savings are possible as well because the

produced coating or filler pigments can be delivered in a slurry form to a paper

mill, possibly through a pipe.

2.2.2.3 Special Pigments

2.2.2.3.1 Talc

Talc as a coating pigment was introduced industrially in Finland and France in

1982. Since then, talcs have become widely used in light weight coated (LWC)

gravure and coated fine papers in offset formulations. Recently another use has

been found for talc in the coating of special papers. In some applications, e.g.

coating paper for gravure printing, talc can technically replace clay if it is economi-

cally viable and vice versa due to the similar platy form of the particles. The charac-

teristics generally differentiating talcs from delaminated Georgia kaolin clays (US)

or Cornish kaolin clays (UK) are hydrophobicity, low cohesion between the crystal

2 Raw Materials for Paper and Board Manufacture58

layers inside the talc particles, resulting in extremely low friction coefficient, soft-

ness and low abrasivity in spite of a large particle diameter. The brightness of

European coating talcs varies from ISO brightness 82 to 88, depending on the

origin.

2.2.2.3.2 Gypsum

Gypsum consists of calcium sulfate crystallised with different degrees of hydra-

tion, all of which can be referred to as gypsum. However, in most cases, gypsum

refers to calcium sulfate in the dihydrate form, and the other forms are distin-

guished from each other by additional names (e.g., hemihydrate, calcined gyp-

sum, and stucco gypsum). The rate of solubility of gypsum of pigment fineness is

high, so the solubility easily reaches equilibrium in, for example, the water circula-

tion of a paper machine. A saturated gypsum solution contains approximately

2.1 g l

–1

of CaSO

, which equals 2.5 g l

–1

as dihydrate and 580 mg l

–1

as a calcium

concentration. Gypsum in itself is a neutral salt that, unlike carbonate, does not

buffer the pH of circulation water. When it comes to solubility, it makes no dif-

ference whether gypsum pigment is used in an acidic or neutral papermaking

process. The major advantages of gypsum pigment include brightness, bulk and

offset printability. Additional optical brighteners used with gypsum are very ef-

fective.

2.2.2.3.3 Plastic Pigments

Plastic pigments are used in paper coatings to provide a surface with the desired

appearance and printability. If properly chosen and formulated, these pigments

provide a coating with surface smoothness, brightness, opacity, as well as a balance

of ink holdout and ink receptivity upon which to print. Organic pigments, com-

monly referred to as “plastic pigments”, are used as partial replacements for in-

organic pigments to improve the optical and print properties of coated paper and

paperboard. There are two general classes of plastic pigments used in the prepara-

tion of coatings for paper and paperboard: solid bead and hollow sphere. Both are

available in a variety of particle sizes and compositions and, in the case of hollow

spheres, in a range of void volumes. All plastic pigments are supplied as polymeric

particles dispersed in water. In the case of hollow-sphere pigments, those particles

are water-filled spheres. During drying the water diffuses through the shell, leav-

ing an air-filled core – hence the term “hollow sphere”. Plastic pigments are used

in paper coatings wherever improvements in finishing efficiency, sheet gloss, or

print gloss are important. They are used extensively in coated folding boxboard,

woodfree coated printing papers, and they are also used in LWC and ULWC pa-

pers.

2.2 Non-fiber Raw Material 59

2.2.2.3.4 Satin White

Satin white (calcium sulfoaluminate) is still of some importance in paper coating.

This pigment has very fine particles; it is extra white, and has a low density. It

increases ink absorption and gloss. The disadvantages of satin white are its sensi-

tivity to increases in temperature and decreases in pH, and its high adhesive de-

mand.

2.2.2.4 Additional Pigments

2.2.2.4.1 Titanium Dioxide

Titanium dioxide pigments show extremely high refractive indices. This, and their

whiteness, i.e., their high reflectance in the visible region of light, as well as their

optimal particle size, make them the most effective white pigments. Titanium

dioxide exists in three crystal forms: anatase, rutile, and brookite. The first two are

stable and therefore of commercial importance. Titanium dioxide pigments are

used in paper coatings to increase the opacity of coated paper. To ensure good

optical efficiency of titanium dioxide in coatings, TiO

pigments must be properly

dispersed in the coating color. Undispersed aggregates or agglomerates diminish

the optical efficiency of TiO

pigments. Rutile slurry pigments are usually the most

effective titanium dioxide grades in paper coatings because they have the highest

TiO

content and refractive index, as well as optimal particle size and particle size

distribution.

References

Section 2.1.4

1 Anon., Papier 2004 – Ein Leistungsbericht,

VDP, Bonn, 2004, 89 pp.

2 Anon., Papier 1993 – Ein Leistungsbericht,

VDP, Bonn, 1993, 64 pp.

3 Anon., Papier 1983 – ein Leistungsbericht,

VDP, Bonn, 1983, 50 pp.

4 Anon., 1972 Annual Review, Pulp Paper

Int. 1972, 14 (8), 67–185.

5 Anon., 1963 Annual Review, Pulp Paper

Int. 1963, 5 (9), 83–212.

6 Anon., Special Recycling 2003 Statistics,

CEPI, Brussels, 2004, 8 pp.

7 Anon., Towards a Sustainable Paper Cycle,

International Institute for Environment

and Development (IIED), London, 1996,

258 pp.

8 Anon., Paper and board – European list of

standard grades of recovered paper and

board. DIN EN 643, Brussels, 2001.

Section 2.2.1

1 M. Altemeier, R. Meyers, F. Aviles, Size

Press Filling Boosts Ash Content, En-

hances Uncoated Free-sheet Quality, Pulp

and Paper, January 2004, 52–54.

2 A. Puurtinen, T. Saari, J. Grön, Laborun-

tersuchungen zur Optimierung des

Schichtaufbaus holzfreier Streichrohpa-

piere, PTS Munich/Germany 09 2002 Con-

ference Proceedings 37–1 to 37–11.

3 M. Laufmann, H. Rapp, Wire Abrasion

and its Potential Causes, Wochenblatt für

Papierfabrikation 1995, 18, 803–812.

4 M. Lex, O. Tamms, Füllstoff Kaoline im

Wandel der Papierherstellung, Wochenblatt

für Papierfabrikation 2003, 5, 233–237.

5 M. Strutz, C. Sweeney, Natural Ground

Calcium Carbonate, Neutral/Alkaline Pa-

permaking Short Course October 1990,

Tappi Reprint

6 M.Laufmann, W. Hummel, Neutral

Groundwood Containing Papermaking,

References

Overview-Anion. Trash-Conversion, Wo-

chenblatt für Papierfabrikation 1991, 119 (6),

269–293.

7 M. Laufmann, W. Hummel, M. Forsblom,

Calcium Carbonate Fillers, in SC Paper

PTS Symposium Munich 2004,

Conference Proceedings, pp. 16/1–

16/18.

8 R. A. Gill, The Effect of PCC Filler Charac-

teristics on the Optical and Permeability

Aspects of Paper, Tappi International

Paper Physics Conference Proceedings,

1991, pp.211–218.

9 K. Doelle, Method of Chemically Loading

Fibers in a Fiber Suspension, US Pat. 6,

355, 138 B1 March12, 2002.

10 S. Middleton, J. Desmeules, A. Scallan,

Lumen Loading with Calcium Carbonate

Fillers, J. Pulp Paper Sci. 2003, 29 (7),

241–248.

11 M. Sanchez, Use of Calcium Sulfate in the

Papermaking Process, PIRA Conference

Barcelona 2003, Scientific and Technical

Advances in Fillers and Pigments for

Papermakers, Conference Proceedings.

12 J. Winkler, Titanium Dioxide, European

Coatings Literature, Hannover Vincentz,

2003, ISBN 3-87870-148-9.

13 M. Pietsch, J. Haverinen, Silicate and

Carbonate Join Forces – A New Filler

Improves Printability, PTS Symposium

Munich 2004, Conference Proceedings pp.

18/1–18/12.

14 P. A. C. Gane, A study of the use of novel

pigments with nanosurface features on

microparticles, Pulp Paper, 2004.

15 T. Harper, Key growth areas and develop-

ments in nanotechnology, Pira Conference

Miami FL USA, 2005, The Future of Nano-

materials Conference Proceedings.

Section 2.2.2

16 E. Lehtinen, Papermaking Science and Tech-

nology, Book 11, Pigment Coating and Sur-

face Sizing of Paper, Fapet Oy, Helsinki

1999, in cooperation with Finnish Paper

Engineers’ Association and TAPPI.

17 L. Huggenberger, Wochenblatt Papierfabrik.,

1999, 16.

18 Calcium Carbonate, From the Cretaceous

Period into the 21

Century, Birkhäuser

Verlag, Basel 2001.

19 E. Eiroma, J. Huuskonen, Pigment Coating

of Paper and Board, Paper Manufacture,

Book 1, Antti Arjas, Ed., Teknillisten tietei-

den akatemia, Turku, 1983.

References